Innovative air vessel design for long distance transmission pipelines

Abstract

The Shuweihat Water Transmission Scheme (SWTS, UAE) consists of a twin DN1600 DI PN25 pipeline transmitting 150 MIGD over 250 km from Shuweihat Desalination plant to Mussafah (Abu Dhabi city). The Scheme is divided in two subsequent systems, each with a tank farm and a pump station delivering water to downstream terminal reservoirs and direct consumers. The first system (Lot A) transmits water from Shuweihat to Mirfa (100 km). The second (Lot C) is from Mirfa to Mussafah (150 km). The pipeline follows roughly the UAE coastline and the profile is generally flat with a few local high points. The surge study of the Lot C system investigated a large number of scenarios and resulted in the design of surge protection equipments and control systems. The surge protection equipment consists of 16 x 121 m3 (1936 m3) innovative vertical non-vented air vessels, invented by Deltares. The innovation includes a passive air release valve at a strategic elevation on the air vessel. This air release valve opens if the water level drops below the float level. At this water level the air pressure is super-atmospheric so that air is released from the air vessel to prevent draining. This innovation was driven by the initial findings of the surge analysis where it became apparent that 22 x 220 m3 (4840 m3) air vessels would have been required. These large vessels were the result of a double constraint. On the one hand, the initial air mass had to be sufficient to allow a proper expansion of the vessels and thus the protection of the pipeline against excessive negative pressures. On the other hand, a very small initial air mass was required due to the lack of backpressure. This meant that the air pocket was expanding by a factor exceeding 20, resulting in the emptying of the vessels. In this paper, we will describe the hydraulic model of the hybrid air vessel, detail the benefits of the new design and discuss the cost savings compared to conventional air vessels. It is concluded that the hybrid air vessel has saved over 50% of the required air vessels total volume. Considering the size of air vessels initially required, the costs savings in terms of surge protection of the SWTS Lot C system were cut in excess of 60%.